The heme oxygenase-1 (HO-1) system consists of the enzyme heme oxygenase-1 and the products of the degradation of heme, carbon monoxide, biliverdin, and iron. Overexpression of HO-1 protects cells and tissues from immune-mediated injury as well as oxidative damage, indicating that HO-1 and its products are an important immunoregulatory mechanism. The mechanisms of this regulation are unknown. We have shown that the absence of HO-1 strongly affects differentiation of resident dendritic cell (DC) subsets in vivo and abrogates regulatory T cell mediated suppression of T cell activation by DC in vitro. These findings support the concept that modulation of the immune response, and therefore allograft rejection, is through the effect of HO- 1 on the differentiation and function of DCs. In support of our overall goal of identification of unique molecular targets for modulation of immune responses, we propose to determine how host HO-1 expression affects the differentiation of resident DC subpopulations and how expression of HO-1 by DCs modulates inflammatory responses in vivo in a renal allograft model of chronic rejection. This objective will be pursued using unique tools developed by the PIs, including i) a transgenic floxed HO-1 mouse strain permitting selective expression of HO-1 by cross-breeding with the appropriate cre recombinants; a CD11c-DC specific HO-1 overexpressing strain; ii) a GFP+ HO-1-/- mouse strain; and iii) an orthotopic murine renal transplantation model of chronic allograft nephropathy (CAN) with features that resemble human CAN. We will address the following specific aims: 1) to test the hypothesis that HO-1 regulates the distribution and differentiation of DC subpopulations in the peripheral lymphoid organs; 2) to test the hypothesis that HO-1 regulates the development of inflammation, fibrosis and vascular disease in CAN by modulation of DC activation and homing in renal allografts; 3) to determine the therapeutic potential of manipulation of HO-1 expression in DCs for kidney allografts by testing the hypothesis that HO-1 overexpressing DC will prolong graft survival, improve renal function and alleviate histological features of CAN. Current research and paradigms with respect to the control of the immune response to solid organ allografts are centered on control of T cell activation and clonal expansion. This approach has been successful in reducing acute allograft rejection, but has not made satisfactory inroads into long-term graft losses due to chronic rejection. The proposed studies are designed to address this need by examining the role of a unique molecular target, heme oxygenase-1 (HO-1), which is part of a group of stress proteins shown to provide cytoprotection. HO-1 is unique among these proteins because of its association with immunomodulation, and protection against fibrosis and vascular disease. Safe, effective therapeutic application of this molecule, effected by manipulation of HO-1 activity or the relative abundance of its substrates and products, requires a much better understanding of its role in immunity and long-term graft survival.